Deployment of high speed transparent and reconfigurable optical networks requires effective, flexible and robust Channel Performance Monitoring (“CPM”) techniques in order to ensure high quality of service as well as high level of resiliency.
The adoption of optical coherent detection, in which the carrier phase and amplitude are recovered at the receiver-side and down-converted to the electrical domain (as opposed to direct detection, in which the phase information is lost), provides an additional degree of freedom to encode and transmit information and consequently offers an improvement in spectral efficiency. Most importantly, this lossless optical-to-electrical signal conversion offers dramatic boost to the applicability of Digital Signal Processing (“DSP”), following high speed analog to digital conversion.
With the shift in technology towards advanced coherent modulation formats and the use of DSP devices, high spectral efficiency optical networks may be designed with almost no restriction on accumulated Chromatic Dispersion (“CD”) and Polarization Mode Dispersion (“PMD”). Current technologies enable compensation of up to +/− 60 000 ps/nm accumulated CD and 30 ps of PMD. Consequently, the transmission reach is limited mainly by the Amplified Spontaneous Emission (“ASE”) noise generated by the optical amplifiers as well as the optical nonlinear effects.
CPM is a requirement set in order to ensure satisfactory signal quality and to provide an in-traffic analysis of the “network health” for the Network Management System (“NMS”). In particularly, CPM enables detecting, reporting and localizing potential failures at the transmission optical link. Exemplary performance parameters that a signal performance monitoring element may provide include (but are not limited to):                1. Accumulated Chromatic Dispersion;        2. Polarization Mode Dispersion;        3. Polarization Dependent Loss (“PDL”);        4. Linear Crosstalk;        5. Nonlinear Crosstalk;        6. Optical Signal to noise Ratio (“OSNR”);        7. Electrical Signal to Noise Ratio (“ESNR”);        8. Optical Signal to noise Ratio Margin;        9. Electrical Signal to Noise Ratio Margin;        10. Overall link impairment strength;        11. Symbol Error Rate (“SER”); and        12. Bit Error Rate (“BER”).        
Several methods have been proposed in the art to derive the In-Band OSNR level by estimating the in band noise level directly, even in the presence of optical filters in the link. These methods comply with the use of polarization multiplexing and coherent optical modulation formats.
A method for In-Band OSNR monitoring based on Stimulated Brillouin Scattering effect has been described in the Applicant's U.S. Pat. No. 8,660,426.
Other methods which rely upon the use of the DSP in a coherent receiver have also been proposed. For example, Z. Dong, A.P.T Lau and C. Lu, in “OSNR monitoring for QPSK and 16-QAM systems in presence of fiber nonlinearities for digital coherent receivers”, Optics Express, vol. 20, no. 17, pp. 19520-19534, 2012, describe a method for fiber-nonlinearity-insensitive OSNR monitoring in digital coherent receivers, which uses incorporating and calibrating fiber nonlinearity-induced amplitude noise correlations among neighboring symbols into conventional OSNR estimation techniques from received signal distributions.
US 20040213338 discloses a method to monitor the optical channel based on Analog to Digital Converter (“ADC”) samples before potential use of an equalizer. The sampled data are conveyed to a DSP unit that may be located at the receiver card or at a remote location, in order to monitor at least one performance parameter of the channel. Furthermore, the publication describes the use of recovered decided data information following a FEC decoder block, in order to increase the signal monitoring capabilities by, for example, separating the histograms of “0” and “1” bits in order to calculate eye opening and signal noise histograms. Therefore, this disclosure relies on a priori knowledge of the characteristics of the specific Forward Error Correction (“FEC”) block that had been deployed, in order to provide channel performance parameters.
U.S. Pat. No. 8,824,902 describes methods for evaluating signal quality within the receiver along the path extending from the A/D convertor to the DSP or within the DSP, using an information extracting circuit, that is able to provide data for a determination means in order to detect Loss of Signal or signal deterioration, for example by analyzing histograms of the recovered constellation, with respect to a given threshold. However, this publication does not relate to the problem which the present invention faces, namely, that there are unknown proprietary techniques used to modulate/demodulate the optical channel. In other words, the recovered constellation is obtained in a non-agnostic way, by relying on known characteristics of the DSP blocks being used. Furthermore, the histogram analysis of the recovered constellation as described in this publication, is not intended for estimating histograms of each constellation point separately, but instead, considers the constellation diagram histogram as a whole.
US 20130236169 discloses dynamic performance monitoring systems and methods for optical networks to extract performance monitoring data in an optical networks based on the monitoring (via the use of a DSP device at the receiver) of existing channels or by using a probe channel with PRBS data before provisioning the service, in order to evaluate the optical path performances. The channel performance monitoring is carried out under real time traffic constraints. This disclosure pre-assumes that the network operator has a control/knowledge of the modulation techniques used for the channel and particularly for the probe channel, so that a known PRBS data stream may be relied upon.
In addition, by compensating different optical link impairments such as CD, PMD and PDL, the DSP unit of a coherent receiver can provide information of the amount of CD, PMD and PDL that the optical signal has undergone.
The ESNR level may also be estimated by comparing the recovered noisy symbols (after passing the DSP block stages before taking a decision) to the decided symbols (after decision has been taken and possible correction via the Forward Error Correction (“FEC”) decoder block was affected).
Monitoring of the OSNR level of the signal is still not sufficient in order to monitor the overall OSNR system margin. Link induced physical degradations, such as received optical power to the receiver, CD, PMD, PDL and more specifically nonlinear effects, might change significantly the OSNR level to be attained for a given BER target and therefore might cause difficulties in the estimation of the overall OSNR system margin. A method for OSNR system margin monitoring, robust to link impairments and based on the evaluation of the ESNR margin with a correction factor has been described in the Applicant's PCT application published under WO 2015132776.
Optical coherent transceivers may be used to provide channel performance parameters derived from a real time DSP block at the receiver (referred to herein as in-line processing approach) that is primarily used to recover the transmitted data at the receiver side. A conventional prior art method for inline processing CPM using DSP and FEC blocks of the coherent receiver is demonstrated in FIG. 1, where the different channel performance monitors (for CD, PMD, PDL, OSNR, ESNR and OSNR margin) are derived from real time processing of the received channel signal, in order to recover the transmitted bit stream. However, such an approach of relying on real time DSP, may be appropriate at the link termination, and is not cost effective for channel performance monitoring purposes since in this case, recovery of real time transmitted data is not necessary. The cost of channel performance monitoring can therefore be reduced by the relaxing the requirement of in-line DSP block and using instead offline processing (at a lower processing rate than the channel symbol rate) of all or some of the DSP function blocks. This cost reduction allows deploying channel performance monitoring elements in strategic optical network nodes in order to get an in-traffic analysis of the “network health”.
Additionally, it is preferable that the DSP based channel performance monitoring should be independent from the DSP and FEC implementation used by a particular coherent transceiver manufacturer, in order to comply with a large number of different transceiver manufacturers. Furthermore, in many cases, the DSP and FEC techniques that are used by the respective transceiver manufacturer, are proprietary information than are not disclosed to network system vendors and/or to network operators.
For example, in order to compensate the optical phase noise, one may employ a differential encoding technique at the transmitter side with an appropriate carrier phase estimation and a compensation technique at the receiver side (for example using the Viterbi & Viterbi algorithm). Information on the differential encoding mapping might not be disclosed to the network system vendor or network operator. If a pilot symbol approach is used to compensate the optical phase noise, information such as the pilot symbol word, overhead and period might not be known, making difficult to impossible for the network system vendor or for the operator to use a similar approach in order to extract channel performance parameters.
Another example of unknown information might be the particular implementation of the FEC encoder and decoder, making very difficult for the network system vendor or operator to acquire the knowledge of the decided symbols after the FEC decoding for ESNR estimation without using the precise FEC algorithm.
Therefore, it would be beneficial to have a channel performance monitoring technique based on a transceiver manufacturer agnostic DSP approach that can overcome such lack of available information associated with a particular DSP and FEC that are being used. In addition, with the shift to network virtualization and software defined optical networks paradigms, it is preferred to develop network element modules that are not restricted by a specific technology or to a specific manufacturer, in order to provide universal features.
Therefore, a method and system that enable low cost channel performance monitoring of an optical communication link that are agnostic to the transceiver manufacturer, are highly desirable.